The present invention relates to an optical waveguide element that functions as a branching element or a wavelength filter.
In recent years, an optical waveguide element that has a waveguide core made of silicon is attracting attention. An optical waveguide that is formed by enclosing a silicon waveguide core in a cladding layer of a material having a lower refractive index than silicon (silicon oxide or silicon nitride, for example) is referred to as a silicon waveguide. The silicon waveguide achieves an extremely high difference in the refractive indices between the core and the cladding layer and thus, if a silicon waveguide is used, a compact optical waveguide element can be realized.
Further, if the optical waveguide element using silicon as the waveguide core uses a silicon on insulator (SOI) substrate, it is possible to form the optical waveguide element using known etching processing, chemical vapor deposition (CVD) etc., which are used in the manufacturing processes of integrated circuits and optical waveguide elements and the like. As a result, there is the advantage that it is not necessary to develop new manufacturing processes. Further, a characteristic of these manufacturing processes is that they are suitable for mass production.
A branching element and a wavelength filter are essential elements when constructing an optical communication system or the like, and known optical waveguide type branching elements include an element that uses a Y-branch waveguide, an element that uses a directional coupler and an element that uses multimode interference (MMI) (refer to JP-T-8-508351 and to E. Kleijn, et al., “Multimode Interference Couplers With Reduced Parasitic Reflections” IEEE Photonics Technology Letters, Vol. 26, pp. 408-410 (2014).)